![]() A known value of I B is applied, and the resulting I C is measured. In addition to leakage tests, the typical transistor tester also checks the β DC. If it is excessive, a shorted collector-base junction is likely. Another leakage current in transistors is the reverse collector-to-base current, I CBO. ![]() A faulty transistor will often have excessive leakage current and can be checked in a transistor tester. This leakage current is usually in the nA range. Ideally I C = 0 but actually there is a small current from collector to emitter, as mentioned earlier, called I CEO (collector-to-emitter current with base open). When a transistor is connected with thebase open (I B = 0), it is in cutoff. Very small leakage currents exist in all transistors and in most cases are small enough to neglect (usually nA). This indicates a defective transistor even before the tester was used, assuming the base-to-emitter voltage is normal. This simple concept emphasizes the importance of point-of-measurement in certain troubleshooting situations. In case 2, if you had taken the initial measurement on the transistor lead itself and the open were internal to the transistor as shown in Below Figure, you would have measured VCC. Importance of Point-of-Measurement in Troubleshooting This situation is illustrated in Below Figure. If you measure on the connecting trace or on the R C lead, you will read V CC. For example, if you measure on the collector lead when there is an external open at the collector pad, you will measure a floating point. The physical point at which you actually measure the voltage is very important in this case. A poor solder joint often results in an open or a highly resistive contact. If the transistor tests good in-circuit but the circuit is not working properly, examine the circuit board for a poor connection at the collector pad or for a break in the connecting trace. The transistor is plugged into the socket on the transistor tester for out-of-circuit tests. An out-of-circuit check of the replacement device is usually a good idea, just to make sure it is OK. If the transistor tests defective, it should be carefully removed and replaced with a known good one. The three clip-leads are connected to the transistor terminals and the tester gives a positive indication if the transistor is good. You can perform an in-circuit check of the transistor using a transistor tester similar to the one shown in below Figure. When components are removed, there is a risk of damage to the PC board contacts and traces. Good troubleshooting practice dictates that you do not unsolder a component from a circuit board unless you are reasonably sure that it is bad or you simply cannot isolate the problem down to a single component. For example, let’s say that an amplifier on a particular printed circuit (PCB) board has malfunctioned. An individual transistor can be tested either in-circuit or out-of-circuit with a transistor tester.
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